Hypoxanthine Phosphoribosyltransferase: A model for 3D analysis using site directed mutagenesis. The Hypoxanthine Phosphoribosyltransferase [HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8] catalyzes the Mg dependent transfer of ribosyl phosphate group from alpha-D-5phosphoribosyl pyrophosphate (PRPP) to the N9 nitrogen of the hypoxanthine or guanine base (Eads et al., 1994). The human enzyme has been subject to extensive investigation since defects in the enzyme are known to cause inherited group and Lesch-Nyhan syndrome (Lesch-et al., 1964). Full length cDNA, encoding the human HPRT has been acquired, amplified and sub cloned into a recombinant expression system specifically designed for the synthesis of high level of soluble, enzymatically active wild type HPRT (kindly provided by Dr. Sydney P. Craig III). The long-term goal of this project is to employ molecular biology and biochemistry to better understand the relationship between structure and function in the HPRT. Site directed mutagenesis and kinetic studies will be used to test the role of amino acids G69, F98 and A191. The amino acids were selected based on the proposed role as structural, catalytic or binding amino acids as seen in the 3D structural model of the human enzyme. Mutagenesis can be used to test the validity of the current 3D model. The information generated by this study can also be helpful for other laboratories in the design of substrate analogs, including xanthine analogs specific against the malarial enzyme.